1. Field of the Invention
The invention generally relates to self-aligned guard ring Schottky diodes and, more particularly, to such diodes wherein the area of the guard ring is reduced to a very accurately controlled minimum.
2. Description of the Prior Art
A self-aligned Schottky barrier diode guard ring which uses a minimum of the device area has been desired for many years. Various techniques have been proposed but leave unsolved the problem of very accurately controlling the minimized surface area of the guard ring in order that reproducible diode characteristics can be achieved. One technique is shown in U.S. Pat. No. 4,063,964 for "Method for Forming a Self-Aligned Schottky Barrier Device Guardring", issued Dec. 20, 1977 in the names of Peressini et al. and assigned to the assignee of the present invention. Said patent relies on the heat induced shrinkage of a metal silicide Schottky barrier-forming metal to delineate the area of a subsequently diffused guard ring. In the paper "A Very Small Schottky Barrier Diode (SBD) With Self-Aligned Guard Ring for VLSI Applications" by Sang U. Kim published in the December, 1979 IEEE Proceedings, p. 49, the area of the guard ring depends upon the extent of lateral undercut of nitride-oxide layers which, in turn, depends upon the etching time of the underlying oxide layer. Neither technique provides optimum area control whereby the smallest area self-aligned guard rings are achieved in a reproducible manner. The IBM.RTM. Technical Disclosure Bulletin, Vol. 21, No. 7, December 1978, pg. 2752 describes a technique for using a boron-doped polysilicon ring for diffusing the guard ring area of a Schottky diode but the polysilicon ring is determined photolithographically and is not of minimum area. Defensive Publication No. T101201, issued Nov. 3, 1981 by Anantha et al. discloses a Schottky diode having no guard ring but which employs an undoped polycrystalline silicon ring which is formed about an isotropically etched hole in nitride-oxide layers, the oxide layer being undercut relative to the nitride layer. The purpose of the ring is to eliminate the "mousehole" (or undercut region) in the oxide prior to placing a Schottky contact inside the ring and on the underlying silicon substrate. The thickness of the polycrystalline ring must be sufficient to fill in the undercut region of the oxide, the dimensions of which can not be closely controlled. If the ring, contrary to the teaching of said copending patent application, had been doped so as to form a guard ring, the area of the guard ring would likewise not be closely controllable.